The purpose of these studies is to clarify the characteristics of Na+, K+ and Cl- transport processes in single medullary (mTALH) and cortical (cTALH) thick limbs of Henle isolated from mouse kidney, and the ways in which these transport processes are modulated by hormones, prostaglandins, peritubular osmolality, CO2 and furosemide. The mTALH absorbs NaCl by furosemide-sensitive co-transport of 1Na+:1K+:2Cl- across apical membranes; conductive Cl- efflux across basolateral membranes; and net Na+ absorption through both a transcellular route and a paracellular route. Apical membranes also contain a K+ transport pathway which recycles lumen to cell K+ uptake back to luminal fluids, and which also results in net K+ secretion. Antidiuretic hormone (ADH), operating via cyclic adenosine monophosphate (cAMP), augments net Cl- absorption and net K+ secretion; prostaglandin E2 (PGE2) antagonizes the action of ADH; and increases in peritubular osmolality with urea block Cl- absorption at a post-cAMP locus. The goals of our studies are to understand the details of Na+, K+ and Cl- transport in mouse mTALH segments, and to describe explicitly the cellular mechanisms by which ADH, PGE2 and peritubular osmolality modify ion transport. The mouse cTALH is functionally heterogeneous from the mTALH. Furosemide-sensitive net Cl- absorption in the cTALH in unaffected by ADH or PGE2, but is enhanced strikingly by (CO2 + HCO3-), although net HCO3- transport is negligibly small. A working model for (CO2 + HCO3-)-stimulated NaCl absorption in the mouse cTALH involves the parallel, synchronous operation of two apical membrane antiport systems: one for Na+ (or another cation)/H+ exchange and one for Cl-/HCO3- (or OH-) exchange. The proposed studies will characterize the requirements of these Na+/H+ and Cl-/HCO3- exchange systems, with particular emphasis on the contributions of apical membrane K+ conductive pathways and basolateral Cl- conductive pathways to net NaCl absorption. The results of the proposed studies should provide information useful in understanding urinary concentrating and diluting mechanisms, and the roles of ADH, PGE2 and peritubular osmolality in modulating these processes.